So I was was looking through the MAME Driver for Taito F2 earlier and I noticed that there's a slightly different machine configuration for the World Release of Dead Connection and the Japanese release of Dead Connection.

I dusted off the PCB and swapped d28-12.5 (world region) for d28-07.5 (jp region) and it BOOTS! My modifications were correct all along.
So apparently there are some differences in the hardware that prevent the single-board World ROMs from working on the 2-board JP hardware. Of course running the JP ROMs now the game is in Japanese :-/

Comparing the ROMs there is literally only 2 BITS different on the last two words of the file
World version ends in 10 03
JP version ends in 00 01

This to me says that both versions of the game are on the same ROM and it's just checking the last byte. Working on the Assumption that one byte is to set the hardware version and the other byte is to set the region... super easy to test in MAME.

running deadconxj in MAME
bits set to 00 01 is the default JP 2-board version in MAME
bits set to 10 03 (just like world version) the game boots in English but the graphics are wrong (sprites out of place)
bits set to 10 01 the game boots in Japanese and the graphics are wrong
bits set to 00 03 the game boots in English and the graphics are correct.

so the second to last word 00 = 2-board version, 10 = single board version
the last word 01 = JP region, 03 = World region.

now I can make a custom 2-board world ROM! but wait... maybe I can make it work for other regions that don't exist?

changing the last word to 00 = USA, Canada, and Mexico region without the winners don't use drugs message
changing the last word to 02 = USA, Canada, and Mexico Region with "Winners Don't Use Drugs" message

I didn't bother playing further than that. But I'm excited that I not only do I now have a working 2-board conversion but that my conversions are now USA region!

UPDATE:
I think the USA Version was never meant to be, the game crashes on me when using that region. works fine with JP or World regions though.

Nice , I gotta attempt this at some point, excellent work guys, thanks for sharing!!

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I've looked at it.
There's a different IO chip, assuming you can make that work, (at minimum you'd have to remap controls, worst case it's a deal breaker). You'd need to increase ROM space like we did here but then you'd need to modify the program ROM to remap everything from address 0x200000 through 0xa00003, basically everything except for the main ROM and RAM space. This includes remapping sprite ram and pallet ram which is a huge amount of work and a huge pain in the ass.

football champ uses a sprite banking feature that metal black doesn't so, that would need to be disabled

Audio CPU also requires a larger ROM, so that would have to be fixed on the hardware as well.

On a whole this conversion would be quite similar to my Koshien to Gun Frontier but with a an extra month worth of mame debugging work to fix the address map. Assuming it even works due to the different IO chip.